Metals introduced during silver halide grain nucleation and/or growth can enter the grains as dopants, and may modify photographic properties of the emulsion grains, depending on their level and location within the grains. When the metal forms a part of a coordination complex, such as a hexacoordination complex or a tetracoordination complex, the ligands can also be occluded within the grains. The presence of such coordination ligands can vary emulsion properties further. The use of dopants in silver halide grains to modify photographic performance is well know in the photographic art, as generally illustrated, e.g., by Research Disclosure, Item 38957, I. Emulsion grains and their preparation, D. Grain modifying conditions and adjustments, paragraphs (3)–(5). Photographic performance attributes known to be affected by dopants include sensitivity, reciprocity failure, and contrast.
Doping with iridium is commonly performed to reduce reciprocity law failure in silver halide emulsions. According to the photographic law of reciprocity, a photographic element should produce the same image with the same exposure, even though exposure intensity and time are varied. For example, an exposure for 1 second at a selected intensity should produce exactly the same result as an exposure of 2 seconds at half the selected intensity. When photographic performance is noted to diverge from the reciprocity law, this is known as reciprocity failure.
Reduced reciprocity failure of silver halide emulsions is important in most, if not all of the silver halide based photographic or imaging systems. Such systems include color and black and white negative film and paper, color reversal film, photothermographic imaging materials, direct x-ray imaging materials, and graphic arts imaging systems. Of course, the time regime over which it is important to have invariant photosensitivity varies from application to application. Thus, doping strategies can depend on the intended use of the doped emulsion. Additionally, since good reciprocity performance is often obtained at the expense of some other desirable photographic response, such as high speed, or negligible latent image keeping, doping strategies are chosen based on the desired feature set of the system in which the emulsion will be used.
In high speed color negative film, it is important to have reduced high intensity reciprocity failure (HIRF) to accommodate short flash exposures and it is important to accomplish this with minimal reduction of film sensitivity (<0.05 log E) for normal exposures, but it is typically not important to have good latent image keeping (LIK) for times less than 24 hours. In color negative paper for optical printing, on the other hand, it is desirable to have both reduced high intensity reciprocity failure and to provide good LIK for times shorter than 24 hours, but it is less important to maintain maximum paper sensitivity.
Iridium salts have long been added to silver halide emulsion grains, at levels typically ranging from about 1×10−9 to 1×10−5 mole/mole Ag, as a means of improving high intensity and low intensity reciprocity failure. Iridium salts of general formula [Ir(X)6-nLn]3−/2− where X=Br or Cl, L=H2O, and n=0, 1, or 2 have been widely used as reciprocity-controlling dopants.
The recognition by Olm et al, U.S. Pat. No. 5,360,712, that metal hexa-coordination and tetra-coordination complexes having at least one organic ligand and at least half of the metal coordination sites occupied by halide or pseudo-halide ligands could be incorporated into the silver halide lattice expanded the number of possible transition metal complexes available for use as dopants for silver halides, including those available for use as reciprocity controlling dopants. This recognition was based on the discovery, described in U.S. Pat. No. 5,360,712, that the selection of the C—C, H—C or C—N—H organic ligands is not limited by steric considerations in the manner indicated previously by Janusonis et al U.S. Pat. No. 4,835,093; McDugle et al U.S. Pat. Nos. 4,933,272, 4,981,781 and 5,037,732; Marchetti et al U.S. Pat. No. 4,937,180; and Keevert et al U.S. Pat. No. 4,945,035. Each of these patents teaches replacing a single halide ion the crystal lattice structure with a non-halide ligand occupying exactly the same lattice position. In fact, the variation of steric forms of C—C, H—C or C—N—H organic ligands observed led to the conclusion that neither the steric form nor size of the organic ligand is in itself a determinant of photographic utility.
U.S. Pat. No. 5,360,712 also teaches that to achieve performance modification attributable to the presence of the organic ligands at least half of the coordination sites provided by the metal ions must be occupied by pseudo-halide, halide or a combination of halide and pseudo-halide ligands. When the organic ligands occupy all or even the majority of coordination sites in the complex, photographic modifications attributable to the presence of the organic ligand were not identified.
The teachings in U.S. Pat. No. 5,360,712 greatly expanded the pool of potentially useful metal coordination complex dopants for silver halide photographic emulsions. With regard to iridium dopants for reciprocity control, U.S. Pat. No. 5,360,712 teaches significant reductions in HIRF are produced by the incorporation as a grain dopant of iridium complexes containing an acetonitrile, pyridazine, thiazole or pyrazine ligand. Additionally, these complexes are capable of significantly reducing LIRF. The synthesis, proof of incorporation and photographic effects of iridium dopants with thiazole ligands were demonstrated in examples describing the dopant K2IrCl5(thiazole).
Kuromoto et al., U.S. Pat. No. 5,462,849 specifically demonstrated that the number of preferred iridium dopants capable of reducing HIRF and LIRF could be expanded still further by use of substituted thiazole ligands or by the use of multiple thiazole ligands. Specific examples of the synthesis of the following iridium dopants were disclosed (tz=thiazole):                MC-49 K[IrCl4 (tz)2]        MC-50 K2 [IrBr5 (tz)]        MC-51 K[IrBr4 (tz)2]        MC-52 K[IrCl4 (H2O)(tz)]        MC-53 K[IrCl4 (4-methylthiazole)2]        MC-54 K2 [IrCl5 (5-methylthiazole)]        MC-55 K[IrCl4 (5-methylthiazole)2]        MC-56 K[IrCl4 (4,5-dimethylthiazole)2]        MC-57 K[IrCl4 (2-bromothiazole)2]An example demonstrating incorporation was shown for MC-54. Examples showing the positive effects of dopants MC-50 to MC-57 on HIRF and LIRF were also shown.        
Mydlarz et al U.S. Pat. No. 6,107,018 describes co-doping silver halide grains with a “class (i)” dopant defined as a dopant capable of providing shallow electron trapping sites and a “class (ii)” dopant which is an iridium coordination complex containing at least one thiazole or substituted thiazole ligand. Co-doping provided greater reduction in reciprocity law failure than could be achieved with either dopant alone and this reduction was beyond the simple additive sum achieved when employing either dopant class by itself. Mydlarz teaches that the thiazole ligands may be substituted with any photographically acceptable substituent which does not prevent incorporation of the dopant into the silver halide grain. Exemplary substituents include lower alkyl (e.g., alkyl groups containing 1–4 carbon atoms), and specifically methyl. A specific example of a substituted thiazole ligand which may be used in accordance with the invention is 5-methylthiazole. Mydlarz also teaches that the class (ii) dopant preferably is an iridium coordination complex having ligands each of which are more electropositive than a cyano ligand. In a specifically preferred form the remaining non-thiazole or non-substituted thiazole ligands of the coordination complexes forming class (ii) dopants are halide ligands. Mydlarz lists the following specific illustrations of iridium thiazole dopants:                (ii-1) [IrCl5 (thiazole)]2         (ii-2) [IrCl4 (thiazole)2]−1         (ii-3) [IrBr5 (thiazole)]−2         (ii-4) [IrBr4 (thiazole)2]−1         (ii-5) [IrCl4 (5-methylthiazole)2]−2         (ii-6) [IrCl4 (5-methylthiazole)2]−1         (ii-7) [IrBr5 (5-methylthiazole)]−2         (ii-8) [IrBr4 (5-methylthiazole)2]−1         
Most iridium dopants are electron-trapping dopants. Electron-trapping dopants affect photographic properties because they trap electrons produced by exposure and then release the electrons. Different electron-trapping dopants can have different electron release profiles, that is, the electrons can be released from the dopant trap over a very narrow time period or a long time period. Additionally, the average time between electron trapping and release can vary from milliseconds to days. As a subclass of electron-trapping dopants, iridium dopants are generally useful in controlling reciprocity because they release electrons in the time frame in which latent image is formed (secs to minutes). The released electrons are incorporated into latent image centers. In iridium-doped emulsions, the time frame of latent image formation is shifted or expanded compared to undoped grains. It is this change in the time frame of latent image formation that leads to reduced reciprocity failure.
In designing an emulsion for a particular product, iridium dopants must be chosen to eliminate reciprocity failure in a time regime appropriate for intended use of the final product. The dopants must also be chosen so as to achieve an optimum trade-off of reciprocity control and with other desirable photographic features such as speed or LIK. As noted above, the final use of the product dictates which photographic features must be balanced with reciprocity control. Quite small changes in electron release profiles and average trapped electron lifetime can affect the final performance dramatically, thus, in optimizing reciprocity performance with other photographic parameters, it is desirable to be able to make small changes in dopant trapping properties. This can be achieved by keeping the central metal ion constant and varying the dopant ligand structure. Referring to iridium dopants with organic ligands, M. T. Olm, R. S. Eachus, W. G. McDugle, R. C. Baetzold state, in Proceedings of the 2000 International Symposium on Silver Halide Technology, Quebec, ISBN: 0-89208-229-1 “These dopants have trapping properties that are not dramatically different from those of (IrCl6)3− and so are useful for improving reciprocity behavior with varying effects on other photographic features.”
The intended use of the final product also dictates the choice of emulsion halide composition. For example, high chloride emulsions are typically used in color paper applications because they develop rapidly. Alternatively, AgBrI emulsions are typically used in color negative film applications because (i) light absorption is enhanced by the presence of iodide for blue-sensitized emulsions, (ii) sensitizing dyes are more readily absorbed onto the AgBrI surfaces compared to AgCl surfaces, and (iii) the presence of iodide in the emulsion allows for partial development of the grain which reduces graininess in the film. The halide composition of the emulsion and the surface morphology can also affect the choice of iridium dopant. As pointed out by Eachus and Olm in J. Soc. Photogr. Sci. Japan Vol. 54, No. 3, p 294-301 (1991), “The lifetime of the impurity center produced by electron trapping is obviously important to the photographic process. It is affected by the identity of the central metal ion, its valence state, the composition of the ligand shell and the composition of the host lattice.”
Based on the teachings and examples of Olm, Kuromoto and Mydlarz, cited above, the most effective organic ligands for use with iridium dopants for reducing HIRF were azoles, with optimum results having been achieved with thiazole ligands. Preferred iridium dopant candidates for reducing reciprocity failure can be chosen from iridium complexes where at least half the ligand shell is comprised of halide ions or pseudohalide ions and the remaining contain at least one thiazole or substituted thiazole ligand. Exemplified compounds have one or two thiazole ligands. Aquated species were also specifically contemplated as demonstrated by compound MC-52. Especially preferred substituents on thiazole ligands were reported to be lower alkyls, specifically methyl. Bromide substituents, as exemplified by compound MC-57, were also specifically contemplated. All of the thiazole substituents are bound to Ir through the nitrogen at position 3. Substitution of the thiazole substituent at the 2, 4 and 5 positions were specifically contemplated as demonstrated by compounds MC-53 to MC-57. Most of these prior art teachings are exemplified by high chloride emulsions designed for color paper products. The only specific examples for high bromide emulsions are for the dopants K4 [Ir2 Cl10(pyz)] and Na3 K2 [IrCl5 (pyz)Fe(CN)5]. These teachings provide some guidance in choosing an optimal dopant for reducing reciprocity failure. However, there are still a large number of possible substituent and location combinations from which to choose.